The ability to move is a fundamental property of almost all types of cells. The best studied mechanism for this conversion of chemical free energy to mechanical motion is the interaction of myosin with actin in skeletal muscle. The object of the proposed research is to improve our understanding of the molecular basis of this process by determining the three-dimensional structure of the myosin domain which interacts directly with actin to generate force and movement. This will be accomplished by providing biochemical support to the study of the structure of the myosin head (s1) by single crystal X-ray diffraction which promises to yield a high resolution three-dimensional structure of the myosin head. To assist n the solution of the crystal structure and in the interpretation of the S1 structure in terms of its function during contraction, the molecular envelope will be determined by electron crystallography of thin sections of the S1 crystals. This envelope will be related by molecular modeling methods to the structure of the muscle crossbridge studied by other electron microscopic and X-ray diffraction techniques. It is generally agreed that the force generating step involves a reorientation of the myosin head with respect to actin. The effect of monoclonal antibodies in myosin ATPase activity, actin binding and on an acto-myosin motility assay will probe the conformational sensitivity of the antibody defined sites. The use of monoclonal antibodies as probes of myosin structure and interactions provides one means of relating the detailed structure of myosin head from crystallography to the dynamic interactions of the muscle crossbridge. The results from this study should advance our understanding of the relationship of molecular structure and cellular function. The simultaneous study of this problem by crystallography and immunochemistry will provide information about the antigenic structure of myosin and the nature of antigen-antibody interactions in general. This may yield valuable insights into the structure of antigenic regions on proteins which will aid in the prediction of immunogenicity for design of synthetic vaccines.